RF-only quadrupole mass spectrometers operating near q=0.908 are very well known, and have certain advantages over mass spectrometers which employ both RF and DC drive voltages. In particular, these RF-only quadrupole mass spectrometers are typically more sensitive than those which employ RF and DC, since in RF-only mass spectrometers, there is no need to be concerned about the effects of the DC on incoming ions, which effects can cause rejection of desired ions. In other words, the acceptance of an RF-only mass spectrometer is typically higher than for an RF/DC mass spectrometer. In addition, the high mass transmission of an RF-only mass spectrometer is typically higher than that of an RF/DC mass spectrometer. The operation of an RF-only mass spectrometer is also usually somewhat simpler, since there is no need to ramp DC with RF. However the peak shape and signal to background of an RF-only mass spectrometer can be more dependant on initial conditions, such as energy and velocity dispersion, and therefore have in the past been generally inferior to that of an RF/DC mass spectrometer.
Various efforts have been focussed on improving the performance of RF-only mass spectrometers, by improving their ability to detect energized ions (e.g. see Peter H. Dawson, U.S. Pat. No. 4,721,854). An alternative approach is discussed in U.S. Pat. No. 5,089,703 issued Feb. 18, 1992. This patent discloses the concept of applying an auxiliary dipole or quadrupole field which excites ions in near resonance with the excitation field by causing them to gain transverse kinetic energy and be rejected, thus producing a notch in the transmission band and allowing derivation of a mass spectrum having improved resolution. The auxiliary field was then modulated in such a way as to permit selective detection of only the modulated ions near q=0.908. However in both cases, this method has the requirement to operate at or just below q=0.908, thereby limiting the mass range of the device.